This invention relates to a motor drive circuit which is especially, but not exclusively, suited to use in an electrical power-assisted steering (EPAS) system for a vehicle.
EPAS systems are known of the kind comprising an electric motor connected through a gearbox to act on a steering mechanism of the vehicle. For example, the gearbox may provide a connection between the motor and a steering column shaft, or directly onto a portion of a rack and pinion mechanism forming part of the steering mechanism.
The electric motor is used to assist a driver in applying torque to the steering mechanism, by applying an assistance torque of the same sense, to make it easier to turn the steering wheel, for example during parking manoeuvres. Thus, operation of the motor may assist in rotating the steering column shaft, or moving a portion of the steering rack mechanism. Of course, the motor may be connected to any part of any typical steering mechanism as long as it can provide an assistance torque to aid the driver in turning the steering wheel.
The motor, which may be a multi-phase brushless star-connected permanent magnet motor, is controlled by motor control means comprising control and drive circuits, which is operative to supply a current from a power supply to the motor phase windings. The phase windings of the motor are connected at a star point. Each phase is connected to a common supply rail which connects in turn to a positive terminal of the power supply by a top transistor, and to a negative supply rail which connects to a negative terminal by a bottom transistor, the two transistors defining an arm of a multiple arm bridge. This bridge forms the drive circuits, while the control circuits are provided by a microprocessor or digital signal processor or analogue signal processing or some combination thereof. The microprocessor is operative in response to signals from a torque sensor provided on the steering column to measure the torque applied by the driver, from a motor rotor position sensor providing information about motor speed and direction and optionally from signals corresponding to current flowing in the motor bridge or power supply. This information can be used in combination with the torque sensor signal and/or column position sensor signal to determine which phase winding should be energised and when. The microprocessor produces control signals which are used by a bridge driver to energise the transistors of the drive circuits to cause current to flow in a desired motor phase.
To prevent noise from the motor being passed to the DC supply terminals, a DC link filter is provided between the terminals and the motor phases. The purpose of this filter is to smooth out or filter out any high frequency noise that may be produced. It typically comprises a capacitor connected between the supply rails, and may also include one or more inductors connected in series in each rail between the terminals and the motor phases.
A problem with this electrical power-assisted steering system is that a fault occurring in the motor drive or control circuits can cause an error condition which is unacceptable in a vehicle steering system, where safety is critical.
For example, suppose that a top transistor in one arm of the bridge is energised in error while a bottom transistor in another arm of the bridge is also energised in error. This fault would result in a phase of the motor becoming permanently energised and cause the motor to become permanently attracted to a particular position, tending to clamp the steering column in position and resist rotation. This would be readily apparent to the driver, and is clearly undesirable. This situation could occur if the bridge driver is at fault, or if a short circuit occurs across the drive stage transistors, or if a short circuit occurs across the DC link supply.
One known solution to this problem is to provide a clutch between the motor and the steering column. The clutch, typically a dog or friction clutch, is normally engaged but in the event of a fault being detected, the clutch is operated to disengage the motor from the steering column. However, the clutch is costly and bulky, and additional test procedures must also be incorporated to check that the clutch can still be disengaged should it be necessary to do so, which again adds to the cost and is time-consuming.
Another solution is to provide a means for isolating the motor drive stage from the motor in the event of a failure, for example by providing a relay between the drive stage transistors and the phase connections to the motor. Yet a further solution is to use a relay to isolate the motor phases from the star point of the motor. Both of these solutions remove the drive current from the motor but are not without their problems and inherent disadvantages. Over time the contact resistance of a relay may deteriorate, perhaps reducing their operational speed, and they are also costly as well as bulky items to add to a motor.
Without any means to isolate the motor from the drive stage the phase windings are still connected together and also still connected to the drive stage. As a result of this an electrically conducting path could still be present around the bridge (i.e. through two short circuited transistors and two phase windings). Then, on rotation of the motor shaft drive due to rotation of the steering column, a back EMF is produced in one or more of the windings. As a result of this EMF and the complete electrical path around the bridge, a substantial current can flow through the motor phase windings which produces a torque in the opposite sense to the rotation of the motor shaft. This resisting torque is highly undesirable because in this case, not only is steering assistance lost due to disconnection of the power supply, but a resisting torque is then applied which makes turning the steering column difficult.
Another problem encountered in such a system is a short circuit to earth across the DC link filter. This is especially problematic where the capacitor is an electro-lytic component as a short can cause a catastrophic failure to occur.
Any device placed in the motor current path needs to be capable of carrying the motor current, breaking the high current inductive loads that may be present, and maintaining a low contact resistance.
These problems are, of course, not unique to applications in electric power steering systems.